Construction of the presently made gas- or oil-burning boilers for the heat supply of industrial plants, caloric centers, public buildings, tenement houses, hotels, etc, was developed by modification and reconstruction of coal-fired boilers.
The original construction was made definitely according to the requirements of coal heating. With the general acceptance of gas and oil heating, the manufacturing firms aimed at making the existing coal-fired construction suitable for oil- or gas heating by minimal reconstruction. Consequently the oil and gas-burning boilers presently do not meet requirements of the up-to-date heating techniques in every respect. During the redesign process of the boilers, the heat dissipation took place first of all through convection, utilization of the radiation energy began at a later stage and this process is still in progress even today. In view of the foregoing it is understandable that certain boiler types, in spite of the mentioned progress hardly deviate from the coal burning constructions.
Two basic types of industrial boiler are distinguished. One of them is a boiler with horizontal furnace, the other one with angular furnace, so-called steep-tube boiler. The systems with low heat output are based on the former one, while those with higher heat output are based on the latter one.
The horizontal boiler system reconstruction amounted only to the removal of the grate and to some modification of the cross section of the flues. The extent of reconstruction indicates that by minimal reconstruction of the boilers designed for coal heating, the conversion to oil- or gas heating could not bring about favorable results. On the other hand, additional disadvantages were the outcome of the above reconstructions, the most important of which is that the traditional flue-tube as constructed does not meet the up-to-date heating and power engineering requirements. At the beginning of the coal heating era, about 80-90 years earlier, the thermal load methods and internal pressure conditions were entirely different in the design of boilers. At that time the power requirements were ensured by the wavy construction. With the use of minimal radiation energy, mainly be convection favorable heat transfer was achieved at lower operating steam pressure, and at lower strength load and smaller wall thickness.
Use of the present up-to-date oil and gas burners offer considerably higher radiation energy. 70-80% of the energy input in the furnace is utilized through radiation and the resultant high thermal load of the furnace surfaces. The proportion of convective heat transfer has decreased from 60-70% to 10-20%. The efficiency has improved, instead of 60-70%, it is up to 80-95%. As a result of these decisive changes the thermal load of the furnace surfaces has significantly increased and the heat distribution has become unstable. The radiation energy may increase even by one order of magnitude along the centerline of the flame starting from the burner in the direction of burning, depending on the heating conditions. At the same time the covective heat transfer both in axial direction and in the horizontal and vertical planes represents a different heat load.
Generally the first part of the flue-tube is subjected to relatively low thermal load, while it is higher in the middle part and excessive in the final part, or in the return band. In addition to the excessive thermal load on a certain part of the flue-tube, due to the boiler system, intensive circulation is not ensured. For this reason the demand for high quality feed-water increases and nonuniform heat extraction appears as an operating condition. This restriction further increases the operation problems and costs.
Further disadvantages of the traditional horizontal boiler systems is that together with the output, the dimension of the structures and the well thicknesses are increasing. The excessive wall thicknesses restrict the improvement of the output. This relationship limits the output range of the boiler. For this reason, operation and manufacture of these structures are not economical at higher output.
A further disadvantage is that the heat transfer coefficient of the surfaces subjected to the highest heat effect, the flue-tube-turn, chamber and tube-wall, is unfavorable.
In contrast with the requirement, the structural element subjected only to convection and to low heat effect i.e. the flue-tube has the more favorable formation with respect to strength. In addition, its heat transfer coefficient and strength factor are even better than those of the structural parts subjected to high thermal load.
The excessively high thermal load, unfavorable cooling, poor heat transfer coefficient and high surface temperature of the radiated surfaces effect the strength factor of the materials as well. This reduces the life of the structures. Life of the original structures was 50 years and that of the present is 15 years. The 70% decline in life is due to the technological drawbacks of the system.
As a result of the drawbacks of the horizontal boiler construction, large apparatus have to be built with only low ouput. There are too many unnecessary built-in parts which represent disadvantages from a functional point of view, machining of the parts requires special machines and skilled workers. Only 50% of the built in surfaces are utilized with respect to the heat transfer. Production of the apparatus is expensive. Their transportation and installation are difficult. They require large space. Their life is only 40%-50% of the expectable.
While the traditional horizontal construction has several drawbacks, it has its advantages as well, in that the furnace is of circular cross section, fitting well the radiation in the radial direction. The thermal loads of the furnace, due to the convection, can be made uniform in radial direction by modifying the extent of radiation.
The confining walls of the more up-to-date boilers with angular furnace and steep tubes are made with membrane walls. These boilers too were developed from the coal variety with significant modifications. The "traditional"however has kept still several unsolved problems, important from a technological point of view with respect of up-to-date heating and power engineering requirements. A further disadvantage of this boiler type is, that with the conversion to gas- or oil heating, life of the boiler is considerably reduced, by about 30-40%. With knowledge of these facts it is necessary to compare the advantages and disadvantages of these boilers.
Disadvantages:
Thermal load of the furnace surfaces is uneven. The het dissipated by radiation varies along the flame centerline. Thermal load of the furnace surfaces, due to the radiation, is minimum in the vicinity of the burner and maximum at the end of the flame, at the rear part and at the partition wall.
The gas fumes arising in the furnace during burning develop at the front of the flame in the minimal and in the middle part in the maximal quantity. Thus with the convective heat transfer the thermal load of the rear furnace surfaces is further increased. Thermal load of the furnace surfaces varies also in radial direction. Density of the infrared heat rays is the maximum where the rays perpendicularly reach the heat receiving surfaces. Thus at the boilers with angular furnace; intensity of the radiation is minimum at the corners and maximum along the line of the planes in the horizontal and vertical axial direction. Moreover, by convection, the maximal thermal load reaches the surfaces at the top.
Neither the natural, nor the forced circulation follow the varying thermal loads of the furnace and its adjustment meets with difficulties. Thus the surface temperature of the parts subjected to critical thermal load is considerably higher than the required value.
The adverse power engineering conditions effect the qualitative requirements of the feedwater as well, resulting in significant investmentincrease. The danger of breakdown exists. Its life is below the expectable by 40-50%. Utilization of the built in surfaces is 60% with respect to the heat output. The unnecessarily parts increase the cost of production.
An advantage of the boiler system with steep tube and membrane wall, is that the strength of the structures is independent from the increase of the output. Thus in the boilers of higher heat output, the wall thickness of the surfaces subjected to the maximum thermal load, strength and heat transfer ratio, is favorable. The ratio of the calorifer and heat dissipating surfaces is greater by 1.7 than that of the horizontal constructions. Its strength ratio, at indentical heat output is 10-times more favorable, than obtainable in the flue-tube-type (horizontal) boilers.